There are many ways to get access to the source code. As a 3rd party developper (user) you can download libmatroska and libebml from there official location in .tar.bz2 format. If you are a Java developer you can checkout JEBML from svn for a Java Matroska reading library.
You can also check out the source code from our SVN repository with the following command:
svn checkout https://svn.matroska.org/svn/matroska/trunk/
If you want to contribute code, you should request a developer access to the source code. And then you just checkout the SVN source code with the following command:
svn checkout https://svn.matroska.org/svn/matroska/trunk/
You will only need your password when applying some changes. You can also browse the source code using our integrated ViewCVS or Subversion tools.